It is well known that underground coal may be gasified and the gasification of the coal process (the UCG process) produces syngas. This process involves the operation of a gasification reactor cavity (the reactor) between parallel horizontal boreholes within a coal seam that is fed with an oxidant gas, examples are air, oxygen, steam or combinations of these gases, through one borehole (the injection well). After ignition of the seam, gasification reactions between the coal and injected oxidant gases form syngas (a mixture of CO, CO2, H2, CH4, and other gasses) and the syngas is removed via the second borehole (the product well).
In the coal gasification process, there are a number of reactions that occur which generate the syngas. Those reactions include:C+H2O=H2+CO (Hetergeneous water-gas shift reaction)CO+H2O=H2+CO2 (Shift conversion)CO+3H2=CH4+H2O (Methanation)C+2H2═CH4 (Hydrogenating gasification)C+½O2═CO (Partial oxidation)C+O2═CO2 (Oxidation)C+CO2=2CO (Boudouard reaction)
In the typical UCG process, as coal is removed by the gasification process, the cavity grows in size and the coal face gradually migrates between the two boreholes as coal is removed by hot gases flowing across the face. When injection gases are fed into the reactor via a liner within the injection well, the emission point of the gas is fixed at the end of the injection well liner. With growth of the reactor, the hot reaction zone of gasification moves away from the injection point of the oxidant gases, which reduces the efficiency of the gasification process resulting in a decline in product quality. There is a known shortening of the injection point process that is known as Continuous Retracting Injection Point (CRIP).
The currently used method to maintain gas quality is to move the injection point of the oxidant gases to match the movement of the coal gasification face, so the injected gases are always accessing fresh coal and product quality is maintained. The movement of the end of the injection well liner is typically achieved by either shortening the liner by cutting off a section of the liner to relocate the delivery point for the oxidant gases, or withdrawing the liner up the injection well which moves the point of injection. The cutting of the injection well liner or withdrawing it from the injection well both achieve re-positioning of the injection point, but require significant logistic operations and specialized equipment operated from the surface, to achieve the objectives. It is desirable to be able to move the injection point of the oxidant gases along with the movement of the gasification face, without the use of devices inserted into the injection well and operated from the surface, such as cutters or liner withdrawal equipment.
Thus, it is desirable to provide a method for automatically shortening a liner for underground coal gasification and it is to this end that the disclosure is directed. This sacrificial liner linkage process for shortening can apply to all UCG activities which require a repositioning of the injection point in a horizontal injection well within the coal seam.